A screw compressor used in an air conditioner, a chiller unit, and the like is used in wide ranges of suction pressures and discharge pressures. Therefore, depending on operation conditions, over-compression is likely to occur in which pressure in a screw rotor tooth groove (a tooth groove space) (pressure in a compression operation chamber) is higher than a discharge pressure. Therefore, in order to reduce the over-compression, for example, a screw compressor described in Patent Literature 1 (Japanese Patent No. 5355336) has been proposed.
The screw compressor described in Patent Literature 1 includes a male rotor (a main rotor) and a female rotor (a sub-rotor) that have substantially parallel rotation axes and rotate while meshing with each other, a casing that houses the male rotor and the female rotor and in which a suction port is formed on a low-pressure side and a discharge port is formed on a high-pressure side, and a volume ratio valve that performs reciprocating movement in a rotation axial direction of the female rotor and the male rotor while sliding with respect to the male rotor and the female rotor. The volume ratio valve is configured to form the discharge port in cooperation with the casing and moves in the axial direction, thereby being capable of changing a volume ratio of a tooth groove space (a compression operation chamber) formed by the male and female rotors and the casing.
In the volume ratio valve, an intermediate port for bleeding pressure in the tooth groove space is provided. When pressure in a discharge chamber is higher than the pressure in the tooth groove space bled from the intermediate port (an insufficient compression state), the volume ratio valve is moved to a discharge side, whereby the discharge port formed by the volume ratio valve is moved further to the discharge side to increase a set volume ratio. Consequently, insufficient compression is corrected.
Further, when the pressure in the discharge chamber is lower than the pressure in the tooth groove space bled from the intermediate port (an over-compression state), the volume ratio valve is moved to a suction side, whereby the discharge port formed by the volume ratio valve is moved to the suction side to reduce the set volume ratio. Consequently, over-compression can be reduced.